Railroad tank cars are designed to transport liquid commodities, gaseous commodities, and commodities that are gas-liquid mixtures. The interior of a tank car is sometimes lined with a material to isolate the structural components of the tank car from the commodity being transported. Tank cars can be insulated or non-insulated, pressurized or non-pressurized, and can be designed for single or multiple loads. Non-pressurized cars have plumbing at the bottom for unloading, and may have an access port and a dome housing with various valving on the top. Pressurized cars can have a pressure plate, valving, and a protective cylindrical dome housing at the top through which loading and unloading can be accomplished.
Various designs of tank cars have been developed for the transportation of specific types of commodities, including for example, foodstuffs and other materials, including hazardous materials that can pose a threat to safety and health if they are spilled. Traditionally, railroad tank cars have been engineered to contain their commodity based on the commodity's physical and chemical properties, and the inherent stresses placed upon the tank car due to those properties. However, in instances of collision and derailment, a tank car can be subjected to additional forces. In recent years, work has been done towards developing standards and criteria for strengthening railroad tank cars to reduce the risk of spills and increase public safety should a train accident occur.
In response to safety concerns, trends in tank car design have resulted in tank cars that are constructed of thicker steels than what would be required based solely upon the structural loading of specific commodities. Current tank cars thus have steel thickness in excess of what is required to retain the commodity pressure and sustain structural loads, and the additional thickness improves the puncture resistance and crashworthiness of the tank car so that the tank car can be less prone to damage. However, the amount of benefit derived from adding thickness to the outer structure of a tank car is limited, and may not suffice to meet desired criteria for avoiding the release of hazardous materials during events such as collisions or derailment.